The disclosure relates generally to additive manufacturing, and more particularly, to methods for aligning calibrated lasers in a multiple laser, metal additive manufacturing system.
Additive manufacturing (AM) includes a wide variety of processes of producing an object through the successive layering of material rather than the removal of material. As such, additive manufacturing can create complex geometries without the use of any sort of tools, molds or fixtures, and with little or no waste material. Instead of machining components from solid billets of material, much of which is cut away and discarded, the only material used in additive manufacturing is what is required to shape the object.
Additive manufacturing techniques typically include taking a three-dimensional computer aided design (CAD) file of the object to be formed, electronically slicing the object into layers, e.g., 18-102 micrometers thick, and creating a file with a two-dimensional image of each layer. The file may then be loaded into a preparation software system that interprets the file such that the object can be built by different types of additive manufacturing systems. In 3D printing, rapid prototyping (RP), and direct digital manufacturing (DDM) forms of additive manufacturing, material layers are selectively dispensed to create the object.
In metal powder additive manufacturing techniques, such as selective laser melting (SLM) and direct metal laser melting (DMLM), metal powder layers are sequentially melted together to form the object. More specifically, fine metal powder layers are sequentially melted after being uniformly distributed using an applicator on a metal powder bed. The metal powder bed can be moved in a vertical axis. The process takes place in a processing chamber having a precisely controlled atmosphere of inert gas, e.g., argon or nitrogen. Once each layer is created, each two dimensional slice of the object geometry can be fused by selectively melting the metal powder. The melting may be performed by a high powered laser such as a 100 Watt ytterbium laser to fully weld (melt) the metal powder to form a solid metal. The laser moves in the X-Y direction using scanning mirrors, and has an intensity sufficient to fully weld (melt) the metal powder to form a solid metal. The metal powder bed is lowered for each subsequent two dimensional layer, and the process repeats until the object is completely formed.
In order to create certain larger objects faster, some metal additive manufacturing systems employ a pair of high powered lasers that work together to form an object. Typically, each laser is individually calibrated so a known offset correction can be applied for each laser, allowing the precise location of the operational field of each laser to be known. In these type machines, as shown in the schematic plan view of FIG. 1, each laser has a field 10, 12 upon which it can create a melt pool on the metal powder on a build platform. The field indicates the entire area upon with any particular laser can work; the laser works within only a small portion of the field at any given time. An overlap region 14 of fields 10, 12 indicates an area in which laser fields 10, 12 intersect or overlap, i.e., both lasers can create a melt pool in that area. Where an outer surface of an object to be created falls within the overlap region, in order to create a smooth outer surface, the lasers must be aligned within the overlap region. That is, the lasers cannot just be individually calibrated, but must be aligned so they work together to create an aligned melt pool in the overlap region. As shown in the enlarged, schematic side view of FIG. 2, layers created by unaligned lasers 16 and 18 create an outer surface 20 of an object 22 in overlap region 14 that is unsmooth or bumpy.
One approach to identify and correct misaligned lasers employs, as shown in the plan view of FIG. 3, a fine tuning scan test using a foil 24 in the location of the build platform having a pair of perpendicular gradations 26, 28 with fine tick marks thereon. The lasers are applied at low power to each gradation and the amount of X and Y mis-alignment is measured. Any alignment correction(s) required is/are applied to the optics of one of the lasers to make the pair of lasers align. This technique however frequently does not correct the mis-alignment sufficiently to create objects with acceptable outer surface smoothness in the overlap region. This technique is inaccurate for a number of reasons. First, the conventional alignment test applies the lasers at low power rather than at the typically higher, operational power that the lasers will be used. Second, the conventional alignment test occurs in a two-dimensional space without consideration of the three-dimensional nature of the actual melt pool. Third, the conventional alignment test employs the relatively imprecise tick marks of the pair of perpendicular gradations, making an accurate alignment correction very difficult to obtain. Finally, the conventional alignment test also does not take into consideration the material that will be used to create the object, and its impact on the melt pool. In order to address the causes of the alignment inaccuracies, conventional laser additive manufacturing systems may also apply an alignment correction randomization in their control software, but this approach is typically ineffective because it only masks the misalignment as opposed to actually correcting for it. Alignment correction randomization works within the laser overlap region (region where multiple lasers can work on the same part on any given layer) by randomizing where each laser starts and stops within the overlapping area preventing the visualization of a single discrete starting and stopping point for each laser along the vertical (Z) axis of the part.